Physical Property Sensor With Active Electronic Circuit And Wireless Power And Data Transmission

ABSTRACT

Wireless sensors configured to sense, record and transmit data are provided herein. In one aspect, the wireless sensor has a power harvesting unit; a transducing A-to-D converter unit, a microcontroller unit, and a transmitting coil. The power harvesting unit is configured to receive an external energizing magnetic field generated by an external interrogator and store the energy generated thereby. The transducing A-to-D converter unit has a passive electrical transducer configured to respond to variations in a physical property and a converter configured to measure a characteristic value of the passive electrical transducer and provide a digital signal that is indicative of the measured characteristic value. The microcontroller unit is electrically coupled to the power harvesting unit and the transducing A-to-D converter unit and is configured to receive the digital signal from the transducing A-to-D converter unit. The transmitting coil is configured to receive the digital signal and to transmit the digital signal to the external interrogator. Upon receiving a start signal, the transducing A-to-D converter unit charges a current through the passive electrical transducer until a reference voltage is reached and, upon reaching the reference voltage, the microcontroller unit receives the digital signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/588,518, filed Aug. 17, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/506,170, filed Jul. 20, 2009, now U.S. Pat. No.8,264,240, which claims priority to and the benefit of U.S. ProvisionalApplication No. 61/082,207, filed Jul. 20, 2008, which applications arehereby incorporated by reference in their entirety.

SUMMARY

In various aspects, the wireless sensor s described herein areconfigured to record and transmit data as well as sense and, optionally,actuate to monitor physical properties of an environment and,optionally, effect changes within that environment. In one aspect, theimplantable wireless sensor can comprise a power harvesting unit; avoltage regulation unit, a transducing oscillator unit, and atransmitting coil. The voltage regulation unit is electrically coupledto the power harvesting unit and is configured to actuate at a minimumvoltage level. The transducing oscillator unit is electrically coupledto the voltage regulation unit and is configured to convert a sensedphysical property into an electrical signal. Also, the transmitting coilis configured to receive the electrical signal and to transmit theelectrical signal to an external antenna.

Optionally, the wireless sensor can be configured to delay theenergizing the voltage regulation unit until a predetermined time delayhas elapsed In one example, a resistive-capacitive (RC) circuit can beused to effect the desired delay.

In a further aspect, the power harvesting unit can comprise an antennaconfigured to receive the external energizing magnetic field; a firstcapacitor coupled to the antenna that is configured to store energy inthe resonance mode; and a means for rectifying and at least doublingvoltage to be stored in a second capacitor for use by the sensor at asubsequent time.

FIGURES

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the instantinvention and together with the description, serve to explain, withoutlimitation, the principles of the invention. Like reference charactersused therein indicate like parts throughout the several drawings.

FIG. 1 is a schematic circuit diagram showing one embodiment of a powerharvesting unit that employs a voltage doubling scheme.

FIG. 2 is a schematic circuit diagram showing one embodiment of acircuit comprising a power harvesting unit and an end of burst detectionunit.

FIG. 3 is a schematic circuit diagram illustrating one embodiment of acircuit having a voltage regulation circuit.

FIG. 4 is a schematic circuit diagram showing one embodiment of acircuit schematic for the RC time-based oscillator unit.

FIG. 5 is a schematic circuit diagram showing a variable capacitor andan IC that is configured to convert capacitance to a digital signal areutilized to form a digital sensor with a transducing capacitor.

FIG. 6 is a schematic circuit diagram showing an MCU controlling thetiming of the rest of the circuit. In addition, the MCU can provide thecontrol signal for the transducing A-to-D converting unit, can store themeasured data into local memory, and can deliver modulated data fortransmission to the interrogator.

FIGS. 7( a) and 7(b) illustrate one embodiment of a PCB-based sensorcircuit encapsulated by a fused silica housing.

FIG. 8 is a schematic circuit diagram of an exemplary embodiment of awireless sensor showing a coil (L1) that is an energy harvesting coiland a capacitor (C 10) or a resistor (R5) that varies predictably withthe measured physical property. In one aspect, a coil (L2) serves as atransmitting coil and a resistor (R1) acts as an attenuator.

FIG. 9 depicts a schematic circuit diagram for an exemplary embodimentof a wireless sensor that uses one coil instead of two.

FIG. 10 depicts the same exemplary circuit as in FIG. 9 but with avoltage doubling scheme.

FIG. 11 is a schematic illustration of an exemplary experimental set-upused in one pressure sensing experiment.

FIG. 12 is a graph showing the results of the pressure sensingexperiment schematically shown in FIG. 11.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, drawing, and claims, and theirprevious and following description. However, before the present devices,systems, and/or methods are disclosed and described, it is to beunderstood that this invention is not limited to the specific devices,systems, and/or methods disclosed unless otherwise specified, as suchcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular aspects only andis not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

As used throughout, the singular forms “a,” “an” and “the” includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to “a capacitor” can include two or more suchcapacitors unless the context indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

The present invention comprises wireless physical property sensorsincorporating active circuitry and systems incorporating the same.Optionally, the sensors can be integrated with a hermetic, unitarypackage. Active circuitry comprises, e.g., integrated circuits (ICs).The inclusion of active circuitry in the electrical design of a wirelesssensor imparts many new traits. It enables the sensor to sense multiplephysical outputs of interest such as, hut not limited to, temperature,flow, stress, strain, and chemical properties. The addition of activecircuitry also decouples the parasitic effects that would otherwisepresent signal processing challenges, allows for precise compensation orcalibration of the sensor and enables some level of real-timestatistical computing, averaging, filtering or combination thereof toensure the statistical fidelity of the information collected by thesensor. The active circuitry can be used to manage the power source beit an on-board temporary (e.g., a capacitor or ultra-capacitor) or apermanent (e.g., a battery) source of energy to operate the sensor in apartially or fully autonomous manner. The active circuitry allows forstorage of multiple sets of information collected by the sensor or usedin processing of the sensor data. Also, the active circuitry enables thestorage of personal information relative to the identity of the hostand/or the sensor. Furthermore, the active circuitry can enable the useof robust protocols and data transmission techniques to communicate withthe interrogator, minimizing the risk of miscommunication andsimplifying the interrogator. Sensors ascending to the present inventionfind widespread use in biomedical, industrial, consumer and automotiveapplications.

Two distinct types of wireless sensors embodying the characteristics ofthe prior paragraphs are disclosed herein: analog and digital sensors.The terms “analog” and “digital” refer to the format of communicationbetween the sensor and the interrogator. The analog sensor generates avariable frequency signal whose frequency precisely represents the datato be retrieved. The digital sensor digitizes information collected bythe sensor prior to transmitting it to the interrogator using some typeof binary modulation such as, without limitation PSK, ASK and FSK.

In one aspect, the analog wireless sensor is comprised of the followingcomponents: a power harvesting unit, an “end of burst” detection unit, avoltage regulation unit, a transducing oscillator unit, and atransmitting coil. Each of these components, as well as their overallassembly and packaging, are exemplarily described below.

The power harvesting unit is comprised of an inductor (L) and acapacitor (C). The inductor couples RF magnetic fields and the capacitorforms a resonant circuit when connected with the inductor and rectifyingdiodes, energy storage capacitors, and over-voltage protection units.The L and C are tuned to the RE magnetic field present with thefollowing equation for maximum energy harvesting.

f=(2*π*(LC)^(1/2))⁻¹  Equation (1)

The number of rectifying diodes used varies according to the geometry ofthe circuit. At least one rectifying diode fir half wave rectificationand a plurality of rectifying diodes for full wave rectification areneeded. In one non-limiting example, a minimum of one rectifying diodefor half wave rectification and four rectifying diodes for full waverectification can be used. Optionally, when higher voltage generation isrequired, a voltage multiplying scheme can be employed, suchimplementation can involve the addition of a number of diodes andcapacitors. The energy storage capacitor employs relatively largecapacitance value compared to other capacitors in the circuit because itis the only energy source for operation of the wireless sensor insequential systems. The value can vary from hundreds of pico Farad totens of microFarad. A Zener diode can be used as an over-voltageprotection unit.

FIG. 1 shows one embodiment of a power harvesting unit that employs avoltage doubling scheme. When the interrogator an generates energizingmagnetic field in proximity to the sensor, the small antenna (Lr) and acapacitor (Cr) pick up the AC voltage in resonance mode. That AC voltageis rectified and charged to a first capacitor C2 through diode D1.Again, the voltage charged in C2 is added to the AC voltage in Lr andCr, and rectified and charged to capacitor Cs through diode D2. Now dueto C2, D1, and D2, the AC voltage available in Lr and Cr is doubled andstored to a second capacitor, Cs, as a DC voltage to be used for sensoroperation at a subsequent time. A zener diode (Dz) may be incorporatedto provide over-voltage protection. This circuit is useful, when thesensor chip used requires high voltage.

In one aspect, in order to recognize the end of the RE magnetic burstaround the sensor, a simple resistive-capacitive (RC) circuit can beused. The capacitor of the RC circuit remains charged while RF field ispresent. As soon as RF field is removed, the capacitor of the RC circuitdischarges through the resistor of the RC circuit with a predeterminedtime delay. In one aspect, the time delay is programmed such thatenergizing RE magnetic field has decayed to a given threshold level.When the RC circuit has discharged, the rest of the circuit “wakes up”tier data transmission—i.e., the discharge of the capacitor triggers aswitch to connect the rest of the circuit to the energy storagecapacitor. Thus, the potential problem of interference of the RE fieldis avoided and the sensor is able to take advantage of the maximum timeavailable for communication.

FIG. 2 depicts one embodiment of a circuit comprising a power harvestingunit and an end of burst detection unit. When the interrogator generatesan energizing magnetic field in proximity to the sensor, the electricalenergy (i.e., voltage) is harvested and stored to capacitor C2. Duringthis energy harvesting time, transistor switch QP is open, so that theenergy being stored is not available to the main sensor circuit and isthus preserved. The main sensor circuit will be connected to the #3 nodeof switch QP later. As soon as the energizing magnetic field disappears,switch QP closes (the #2 and #3 nodes of QP are electrically connected),and the voltage in C2 is available to power the sensor.

The detailed explanation of the circuit depicted in FIG. 2 is asfollows: the L1 and C3 pick up the AC voltage which is doubled andrectified to C2 through C6 and two diodes in BAS4OBRW (#1 and #5 node inBAS4OBRW). Simultaneously, the AC voltage in L1 and C3 is doubled andrectified to C11 through C7 and two diodes in BAS4OBRW (#2 and #4 nodein BAS4OBRW). Due to the voltage in C11, the switch QP is open and theenergy in C3 is not available to the #3 node of switch QP. When theenergizing field from the interrogator is off, the voltage in C11dissipates through resistor R and becomes zero in short time (i.e., lessthan a millisecond). As soon as the voltage in C11 becomes zero, theswitch QP closes and the voltage in C2 is available to main sensorcircuit.

Thus, in one exemplary embodiment, the power harvesting unit is aLC—tank that is followed by a rectification stage and an optionalvoltage multiplication stage. It is then an energy storage stage, suchas a capacitor with a desired energy storage space.

Most of electronic oscillators call for precise control of supplyvoltage for accurate operation. This is true for the RC oscillator usedin this scheme. Because the voltage source in the analog wireless sensoris a capacitor, the supply voltages decay unless the capacitance valueis impractically large. A linear voltage regulator and a voltagereference can be used for its preciseness and simplicity. Since thelinear voltage regulator and the voltage reference do not controlvoltage below their rated voltage, an under-voltage lockout is employedto avoid uncontrolled operation under the rated voltage of the linearvoltage regulator and voltage reference by turning off the subsequentcircuit when the voltage drops below the predetermined value.

FIG. 3 illustrates one embodiment of a circuit having a voltageregulation circuit. The unit V1 is a voltage reference and U2 is anunder-voltage lockout unit. C8 is noise reduction capacitor. The #1 nodeof V1 is connected to #3 node of QP switch, therefore the voltage storedin C2 is available to the #1 node of V1. V1 functions to output a presetvoltage level whenever the input (#1) voltage is higher than presetoutput voltage. This ensures the voltage available to sensor transducerunit is substantially constant The sensor transducer unit is connectedto output (#2) of V1. The constant level of voltage contributes to theaccurate functioning of the transducer unit. However, if the inputvoltage (#1 node) of V1 is lower than preset output voltage (#2 node),the output voltage of V1 follows the input regardless of the presetoutput. This voltage variation can add noise to sensor transducerfunction when it is supplied to transducer unit. In order to manage thislow-voltage faulty condition, U2 is employed. U2 monitors voltage in C8and, if the voltage in C8 is smaller than preset voltage, it generates awarning or disabling signal through #6 node of U2. The preset voltagelevel is adjusted with resistor values in R2, R4, and R6. When thevoltage is higher than the preset voltage, then the preset voltage isavailable to #6 node of U2. When the voltage in C8 is lower than thepreset voltage, the zero voltage is available to #6 node of U2, eitherwarning or disabling transducer unit.

Next, the transducing oscillator unit converts physical properties suchas, but not limited to, pressure, temperature, stress and the like toelectrical signals. Subsequently, the electrical signals set theelectrical oscillation frequency, which is transmitted to theinterrogator. This electrical frequency can be measured and correlatedto the value of the measured physical property by the interrogator.

The RC time constant-based oscillator can be used for setting theelectrical oscillation frequency. One example is a 555 timer. In oneaspect, the relationship between frequency and the sensed property isdescribed with the equation below:

f=(kRC)⁻¹,  Equation (2)

where k is a proportional constant varying by design and operationfrequency, R and C are, respectively, the resistor and capacitor valuesthat sense the physical property of interest. It is contemplated thateither one of the R or C values can be held constant and the other canbe used to sense the physical property of interest.

FIG. 4 shows one embodiment of a circuit schematic for the RC time-basedoscillator unit. In this exemplary aspect, a capacitor (C10) or aresistor (R5) represents the sensing capacitor or sensing resistor. The#4 node is an operating-voltage supply line, and #3 node is achip-function enabling line. In this aspect, if zero voltage is suppliedto #3 node, the whole chip is configured to shut down. In a furtheraspect, the line that is in communication with the #3 node can also beelectrically coupled to the #6 node of U2 in the FIG. 3, which acts toshut down the chip when the voltage is lower than the preset voltage. Inone aspect, the exemplary circuit is configured such that the oscillatorfrequency can be varied by the value of C10 and R5 and can be availablethrough the #5 node in order to be transmitted back to the reader orinterrogator wirelessly.

Conventionally, a passive (no battery) LC resonant circuit is composedof two electrical passive components that are connected in series: acoil or inductor (“L”), and a capacitor (“C”). Such a passive electricalcircuit exhibits electrical resonance when subjected to an alternatingelectromagnetic field. In one aspect, the electrical resonance isparticularly acute for a specific frequency value or range of theimpinging signal. When the impinging signal substantially reaches theresonant frequency of the LC resonant circuit inside the sensorassembly, a pronounced disturbance of the field can be detectedwirelessly. In the simplest approximation, the electrical resonanceoccurs for a frequency f, related to the value of L and C according toequation 1 above.

The passive electrical resonant circuit for the assemblies describedherein that utilize a passive electrical resonant circuit can befabricated via conventional MEMS approach to sensor design, which lendsitself to the fabrication of small sensors that can be formed usingbiocompatible polymers as substrate materials. In a further aspect,appropriately biocompatible coatings can be applied to the surfaces ofthe respective assemblies in order to prevent adhesion of biologicalsubstances to the respective assemblies that could interfere with theirproper function.

In one example, it is contemplated that the passive electrical resonantcircuit of the assembly can be manufactured using Micro-machiningtechniques that were developed for the integrated circuit industry. Anexample of this type of sensor features an inductive capacitive (LC)resonant circuit with a variable capacitor is described in Allen et al.,U.S. Pat. No. 6,111,520, which is incorporated herein by reference. Inthis sensor, the capacitance varies with the pressure of the environmentin which the capacitor is placed. Consequently, the resonant frequencyof the exemplary LC circuit of the Allen pressure sensor variesdepending on the pressure of the environment.

In one aspect, to convert stress to resistance, piezoresistive materialcan be used to transduce sensed stress to resistance. An example of thistype of sensor is exemplarily described in Gershenfeld et al., U.S. Pat.No. 6,025,725, which is incorporated herein by reference.

In one aspect, to convert temperature to resistance, a thermistor can beemployed. In this aspect, the thermistor is defined as a metal whosespecific electrical resistance varies according to the temperature of amaterial. The environmental temperature determines the resistance which,in turn, determines the oscillation frequency of the RC oscillator unit.In this aspect, the thermistor is attached to the sensor housing forbetter thermal conduction.

Thus, it is contemplated that the sensor can be configured to convert asensed physical property into an electrical signal comprises. Forexample and without limitation, this conversion can comprise convertingsensed temperature changes to resistance changes via a temperaturesensitive resistor. Further, for example and without limitation, thisconversion can comprise converting sensed changes in pressure to changesin capacitance values via a pressure sensitive capacitor. In anotherexample, for example and without limitation, this conversion cancomprise converting sensed stress changes to resistance changes via astress sensitive resistor.

Optionally, a voltage controlled oscillator (VCO) can also be used fortransducing a physical property of interest to an electrical signal. Aconventional VCO is an electronic circuit whose electrical oscillationoutput is a function of the voltage applied. In this aspect, themeasured physical property is converted to a voltage that determines thebroadcasting electrical frequency. Many physical properties are readilyconverted to voltage. For example, and without limitation, suchconvertible physical properties include measured, temperature, stress,pressure or the like.

In one exemplary aspect, to accomplish a temperature to voltagetransduction, a constant current source can be applied to a resistor.Here, the induced voltage at the end of the resistor is proportional tothe value of the resistor. Therefore, temperature change can beconverted to voltage through a thermistor.

In an additional exemplary aspect, to accomplish a stress or pressure tovoltage transduction, four piezoresitors can be arranged into aWheatstone bridge configuration, which is configured to achieve a highervoltage sensitivity while geometrically compensating for temperatureeffects. In this aspect, the stress applied by the external environmentto the deformable region of the sensor determines the output voltage ofthe Wheatstone bridge, and which sets the electrical oscillation forbroadcasting.

Optionally, it is contemplated that an identification signal orsignature can be added to the transmitted signal. In order to attachsuch a signature, the oscillation frequency can be amplitude modulatedprior to transmission to the interrogator. Subsequently, theinterrogator can confirm this signature and authenticate that thefrequency is coming from a specific wireless sensor or merely thewireless sensor at all. This signature modality allows for sensoridentification and avoids any confusion with other resonances present inthe environment.

In one aspect, a digital wireless sensor is comprised of the followingcomponents: a power harvesting unit, an end of burst detection unit, amicrocontroller unit (MCU), a transducing analog-to-digital converterunit, and a transmitting coil. Optionally, a voltage regulation unit canbe incorporated into the digital wireless sensor. The design of thepower harvesting unit, the end of burst detection unit and the voltageregulation unit are substantially the same for the digital sensor asdescribed above for the analog sensor.

As shown in FIG. 5, a variable capacitor and an IC that is configured toconvert capacitance to a digital signal are utilized to form a digitalsensor with a transducing capacitor. In operation, as indicated in FIG.6, the MCU sends out a signal to start the frequency generator and thecounter. Substantially simultaneously, the capacitance is charged at acontrolled rate. In one non-limiting example, a constant current sourceor a constant voltage source can be used to charge the capacitor. Oneskilled in the art will appreciate that the constant current source doesnot require a resistor can also produce a more linear digital outputover a wide range of capacitance.

When the voltage of the capacitor reaches the reference voltage of thevoltage regulator, the comparator trips and the counter is turned off.The MCU reads the digital output and discharges the capacitor inpreparation for the next measurement. In one aspect, the digital outputis a number that is substantially proportional to the capacitance value.In a further aspect, the digital output resolution depends on thefrequency of the frequency generator, which should be kept high enoughto support resolution of the capacitance value necessary for meaningfuloutput to the end user of the system.

As schematically shown in FIG. 6, the MCU controls the timing of therest of the circuit. In addition, the MCU can provide the control signalfor the transducing A-to-D converting unit, can store the measured datainto local memory, and can deliver modulated data for transmission tothe interrogator. Conventional off-the-shelf MCU chips typicallycomprise a frequency generator, a counter, and a comparator with voltagereferences, and can realize the schematic in FIG. 6 with minimalcomponents added.

FIG. 8 shows a schematic circuit diagram of an exemplary embodiment of awireless sensor. As mentioned previously, a coil (L1) is an energyharvesting coil and a capacitor (C10) or a resistor (R5) variespredictably with the measured physical property. In one aspect a coil(L2) serves as a transmitting coil and a resistor (R1) acts as anattenuator.

FIG. 9 depicts a schematic for an exemplary embodiment of a wirelesssensor that uses one coil instead of two. In one aspect the plurality ofinductor coils employed in the previous examples are not functionallyactive simultaneously, i.e., the transmit coil is not in use while theenergy pick up coil is active and vice-versa. Therefore, a single coilwireless sensor is achievable with certain modifications. In thisaspect, the output of the transducer oscillator can be connected to thecoil (L1) through MOSFET switch M3 whenever the transducer oscillatoractivates and transmits a signal. Thus, coil L1 can be used for bothenergy harvesting and signal transmission. FIG. 10 depicts the samecircuit as in FIG. 9 but with a voltage doubling scheme.

In one aspect, the packaging of the sensors can comprise encapsulatingPCB-based sensor circuits in ceramic housings. For example and withoutlimitation, the ceramic housing can be completed either via laserfusion, anodic bonding or eutectic bonding. It is of course contemplatedthat the PCB-based sensor circuit can be replaced with an ASIC and thisreplacement is within the scope of the present invention. In variousexemplary aspect, the sensor can be packaged by the methods disclosed inU.S. patent application Ser. No. 11/472,905, filed Jun. 22, 2006; U.S.patent application Ser. No. 11/314,046, filed Dec. 20, 2005; U.S. patentapplication Ser. No. 11/157,375, filed Jun. 21, 2005; all of which areincorporated herein by reference in their entireties. In one aspect,FIGS. 7( a) and 7(b) illustrate one possible embodiment of a PCB-basedsensor circuit encapsulated by a fused silica housing.

In one aspect, in operation, it is contemplated that the external sourceof the energizing magnetic field, such as an interrogator, will transmitenergy at a substantially constant preselected frequency. In a furtheraspect, the external source will activate or energize for apredetermined period of time, during which time the power harvestingunit of the wireless sensor is energized. After the predetermined periodof energizing time has passed, energy accumulated in the powerharvesting unit is supplied to the voltage regulation unit and issubsequently supplied to the transducing oscillator unit at thesubstantially constant preset voltage level output from the voltageregulation unit. Subsequently, the electrical signal indicative of thesensed physical property is transmitted via the transmitting coil of thesensor to a remote, external antenna, which can form a portion of theinterrogator.

FIG. 11 shows the brief illustration of an exemplary experimental set-upused in one pressure sensing experiment. In this experiment, thecapacitor (C 10) in the schematic illustrated in FIG. 8 is replaced witha pressure sensitive capacitor. Pressure was applied to the sensor via ahand pump.

In this experiment, the timing control unit sent out periodic triggering(in/off) signal to RF power amp. A strong 13.56 MHz signal was turnedon/off at a rate of 35 Hz and subsequently feed to the antenna. Here,whenever the 13.56 MHz signal is turned off, the wireless pressuresensor “wakes up” and transmits the pressure information in the form ofelectrical frequency. A receiving antenna sensed the transmittedfrequency by wireless sensor, and then the received signal was amplifiedand feed to the frequency counter. The results of the experiment areshown in FIG. 12.

These and other modifications and variations to the present disclosurecan be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present disclosure, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments can beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only and is not intended to limit the disclosure sofurther described in such appended claims.

What is claimed is:
 1. A wireless sensor, comprising: a power harvestingunit configured to receive an external energizing magnetic fieldgenerated by an external interrogator and store the energy generatedthereby; a transducing A-to-D converter unit comprising a passiveelectrical transducer configured to respond to variations in a physicalproperty and a converter configured to measure a characteristic value ofthe passive electrical transducer and provide a digital signal that isindicative of the measured characteristic value; a microcontroller unitelectrically coupled to the power harvesting unit and the transducingA-to-D converter unit, wherein the microcontroller unit is configured toreceive the digital signal from the transducing A-to-D converter unit;and a transmitting coil configured to receive the digital signal fromthe microcontroller unit and transmit the digital signal to the externalinterrogator; wherein, upon receiving a start signal, the transducingA-to-D converter unit charges a current through the passive electricaltransducer until a reference voltage is reached and, upon reaching thereference voltage, the microcontroller unit receives the digital signal.2. The wireless sensor of claim 1, wherein the power harvesting unitfurther comprises: a resonant circuit having a resonant frequencydefining a resonant mode of the power harvesting unit; and an energystorage sub-unit electrically coupled to the resonant circuit.
 3. Thewireless sensor of claim 2, wherein the power harvesting unit furthercomprises a rectification sub-unit electrically coupled to the resonantcircuit and the energy storage sub-unit.
 4. The wireless sensor of claim3, wherein the rectification sub-unit comprises at least one rectifyingdiode and at least one rectifying capacitor electrically coupled to theresonant circuit.
 5. The wireless sensor of claim 2, wherein the energystorage sub-unit further comprises an energy storage capacitor.
 6. Thewireless sensor of claim 5, wherein the power harvesting unit furthercomprises a voltage multiplication sub-unit electrically coupled to theresonant circuit.
 7. The wireless sensor of claim 6, wherein the voltagemultiplication sub-unit comprises at least one rectifying diodeelectrically coupled to the resonant circuit and the energy storagecapacitor.
 8. The wireless sensor of claim 1, wherein the sensor furthercomprises an end of burst detection unit electrically coupled to thepower harvesting unit and the microcontroller unit, wherein the end ofburst detection unit is configured to delay energizing at least themicrocontroller until a predetermined time delay has elapsed.
 9. Thewireless sensor of claim 8, wherein the end of burst detection unitcomprises a resistive-capacitive (R-C) circuit configured to provide thepredetermined time delay, and wherein, upon discharge, the R-C circuitis configured to close a switch, thereby electrically coupling the powerharvesting unit and the microcontroller unit to allow energy stored inthe energy storage sub-unit to energize at least the microcontrollerunit.
 10. The wireless sensor of claim 1, wherein the power harvestingunit further comprises a Zener diode electrically coupled to the energystorage sub-unit, wherein the Zener diode is configured to provideover-voltage protection to the power harvesting unit.
 11. The wirelesssensor of claim 1, wherein the passive electrical transducer comprises avariable gap capacitor.
 12. The wireless sensor of claim 1, wherein thepassive electrical transducer comprises a temperature-sensitiveresistor.
 13. The wireless sensor of claim 1, wherein the passiveelectrical transducer comprises a stress-sensitive resistor.
 14. Thewireless sensor of claim 1, wherein the physical property is at leastone of pressure, stress, strain, and temperature.
 15. The wirelesssensor of claim 1, wherein the converter further comprises: a frequencygenerator electrically coupled to the microcontroller, wherein thefrequency generator is configured to activate upon receipt of the startsignal; a counter electrically coupled to the microcontroller and thefrequency generator, wherein the counter is configured to activate uponat least one of receipt of the start signal and activation of thefrequency generator; and a comparator electrically coupled to thepassive electrical transducer and the counter, wherein the comparator isconfigured to deactivate the counter when the reference voltage isreached.
 16. The wireless sensor of claim 1, wherein the microcontrollerunit is configured to append an identification signal to the digitalsignal prior to sending the digital signal to the transmission coil,wherein the identification signal is unique to at least one of a hostand the sensor.
 17. The wireless sensor of claim 1, wherein themicrocontroller unit is configured to modulate the digital signal via,the transmitting coil with at least one of frequency-shift-keying,amplitude-shift-keying, and phase-shift-keying.
 18. The wireless sensorof claim 1, further comprising a voltage regulation unit having a presetactivation voltage, wherein the voltage regulation unit is electricallycoupled to the power harvesting unit and the microcontroller unit andwherein the voltage regulation unit is configured to maintain asubstantially constant voltage level.
 19. The wireless sensor of claim18, wherein the voltage regulation unit comprises: a voltage referenceunit that outputs a substantially constant preset voltage level when thereceived voltage from the power harvesting unit is higher than thepreset activation voltage; and an under-voltage lockout unit that iselectrically coupled to the voltage reference unit.
 20. The wirelesssensor of claim 1, wherein at least a portion of at least two of thepower harvesting unit, the transducing A-to-D converter unit, themicrocontroller unit, and the transmitting coil are integrated into asingle integrated circuit.
 21. A wireless sensor, comprising: a powerharvesting unit configured to receive an external energizing magneticfield generated by an external interrogator and store the energygenerated thereby; a transducing A-to-D converter unit comprising apassive electrical transducer configured to respond to variations in aphysical property and a converter configured to measure a characteristicvalue of the passive electrical transducer and provide a digital signalthat is indicative of the measured characteristic value; and amicrocontroller unit electrically coupled to the power harvesting unitand the transducing A-to-D converter unit, wherein the microcontrollerunit is configured to receive the digital signal from the transducingA-D converter unit; and wherein, upon receiving a start signal, thetransducing A-to-D converter unit charges a current through the passiveelectrical transducer until a reference voltage is reached and, uponreaching the reference voltage, the microcontroller unit receives thedigital signal; and wherein the power harvesting unit is furtherconfigured to receive the digital signal from the microcontroller unitand transmit the digital signal to the external interrogator.